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Background: Preclinical evidence suggests that retinoids and antioxidants may prevent or delay the occurrence of cancer in the upper or lower airways, but such effects have not been reliably established in clinical studies. To assess the chemopreventive effects of vitamin A (retinyl palmitate) and N-acetylcysteine, we conducted a large randomized intervention study in patients with head and neck cancer or with lung cancer, most of whom had a history of smoking. Methods: From June 1988 through July 1994, a total of 2592 patients (60% with head and neck cancer and 40% with lung cancer) were randomly assigned to receive 1) retinyl palmitate (300000 IU daily for 1 year followed by 150000 IU for a 2^nd year), 2) N-acetylcysteine (600 mg daily for 2 years), 3) both compounds, or 4) no intervention. All statistical tests were two-sided. Results: Of the patients, 93.5% had smoked tobacco at sometime in their …